Mean operator requires an adjacent entity of higher dimension

Hi,
I am modelling a 3D DC discharge plasma model,
My model is a lil bit complex and i have followed the same steps as a 2D DC discharge plasma model.
Everything is built but when I try to compute the same it gives the error displayed below.
Failed to evaluate operator.
- Operator: mean
- Geometry: 1
- Boundary: 1 2 3 4 12 13 16 17 18 19 20 21 38 39 40
Mean operator requires an adjacent entity of higher dimension.
Failed to evaluate expression.
- Expression: mean(comp1.dc.T)
Can anyone please explain as under what circumstances this kind of error pops up.
Thanks in advance